Descriptions of memory card technology in popular electronic journals, and in technical training establishments, have greatly contributed to attempts at making fraudulent use of telephone sets by emulating electronic memory cards or "phone cards" designed to operate with such telephone sets. Furthermore, the widespread commercial availability of highly technical components, such as programmed microprocessors, and the ease with which they can be used as acquired in school laboratories or from the programmers of microprocessors, means that this type of fraud can be expected to increase.
There exist two broad categories of phone card simulators, namely:
wired 1 gic simulators known as "passive" simulators which imitate a phone card by connecting together discrete components; and PA1 programmed logic simulators, known as "active" simulators, which are devices essentially comprising a programmed microcontroller associated with a few passive components (resistors, capacitors). By means of an internal program, the microcontroller simulates the behavior of a phone card. Components of that type are now widely available on the non-professional market, as are tools for programming and development, in particular in the form of software running on personal computers.
In general, phone card fraud consists in connecting the active or passive simulator to the electrical contact zones of the electronic module of the card via connection wires, so as to connect the electronic simulator to the telephone set. In order to detect that type of fraud, the telephone set is provided with a specific device known as an external connection detector (ECD). The detector device is mainly constituted by a large and plane electrode suitable for overlaying the connection wires connecting the simulator to the contact zones. When there are no fraudulent connection wires, the only electrical capacitance present between the card and said electrode is a capacitance C0 due to the environment of the card and the electronics. However, the presence of fraudulent connection wires creates a distributed capacitance C1 between the plane electrode and the contacts of the pay phone terminal. An ECD is thus based on the principle of detecting an increase in capacitance from C0 to C0+C1 when a phone card is inserted having connection wires fitted thereto and leading to a simulator.
ECD detection suffers from the drawback that the extra capacitance C1 drifts with temperature and humidity, which means that in order to obtain best performance in terms of number of wires to be detected, it must have automatic correction based on hardware or semi-automatic correction based on software. It is then advantageous to make use of a differential circuit so as to cancel out the effects of the drift in the extra capacitance.